Heating, ventilation and air conditioning (HVAC) systems typically recirculate air, exhaust a portion of the re-circulating air, and simultaneously replace such exhaust air with fresh air. In order to maintain an air temperature and humidity level within a certain space at or near a set point, it is desirable to condition the fresh air the temperature and humidity level set point. Unfortunately, the temperature and humidity of fresh air often differ substantially from those of the set points. For example, during hot and humid periods, such as the summer months, the incoming fresh air typically has a higher temperature and/or humidity level than desired. Additionally, during cold and/or dry periods, such as the winter months, the incoming fresh air typically has a lower temperature and humidity level than desired. The HVAC system must, therefore, condition the fresh air before introducing it to the room.
HVAC systems are typically designed according to the worst climatic conditions for the geographic area in which the HVAC system will be located. Such worst case climatic conditions are referred to as a cooling and heating “design day.” Conditioning the fresh air during such extreme climatic conditions creates a significant load on the HVAC system. System designers, therefore, typically design the HVAC system with sufficient capacity to maintain the set point during the design day conditions. In order to create the required capacity, the HVAC system may include oversized equipment. Alternatively, as discussed in U.S. Pat. No. 4,051,898, which is hereby incorporated by reference, in order to reduce the load on the HVAC system, system designers often incorporate ventilators within the HVAC system. Reducing the ventilation load on the HVAC system decreases its capacity requirements, which, in turn, allows the designers to specify smaller sized equipment, thereby leading to a more efficient design.
Referring to FIG. 1, a ventilator 10 typically includes a plate-type heat exchanger 12 which creates alternating flow passages for the fresh air stream and exhaust air stream to pass therethrough. The flow passages are typically either parallel or perpendicular to one another. This figure illustrates a cross flow heat exchanger because the alternating flow passages are perpendicular to one another. Specifically, one air stream enters the ventilator 10 through opening 11, passes through the plate-type heat exchanger 12, and exits the ventilator 10 through opening 13, and the other air stream enters the ventilator 10 through opening 15, passes through the plate-type heat exchanger 12, and exits the ventilator 10 through opening 17. However, if the alternating flow passages are parallel to one another and the air streams are in the same direction, then the heat exchanger is referred to as a co-flow heat exchanger. Additionally, if the alternating flow passages are parallel to one another but the air streams directly oppose one another, then the heat exchanger is referred to as a counterflow heat exchanger.
Regardless of the direction of the flow patterns, as the air streams pass through the passageway and along opposite sides of the plates, the heat or energy in one air stream is transferred to the other air stream. Depending upon the material of the plates 20, they can transfer sensible heat or both sensible and latent heat. Specifically, if the plates 20 are constructed of a material that is only capable of transferring sensible heat, then the ventilator is referred to as a heat recovery ventilator (HRV). If, however, the plates 20 are constructed of a material that is capable of transferring latent heat, as well as sensible heat, then the ventilator is referred to as an energy recovery ventilator (ERV). For example, metal plates, such as aluminum plates, absorb a portion of the thermal energy in one air stream and transfer such energy to the other air stream by undergoing a temperature change without allowing any moisture to pass therethrough. Therefore, a ventilator constructed of metal plates is referred to as a HRV. Although plates 20 constructed of paper typically have a lower thermal conductivity than metal, paper may be capable of transferring some sensible heat. These plates, however, are capable of transferring some latent heat because such materials are capable of transferring moisture between air streams. A ventilator having plates constructed of material capable of transferring moisture between air streams is, therefore, referred to as an ERV.
It is generally understood that an ERV is more versatile and beneficial than an HRV. However, materials such as paper limit the plate's ability to transfer a larger portion of the latent heat from one air stream to the other air stream. Therefore, it is desirable to produce an ERV with a plate having a greater latent heat transfer efficiency. The cost of the more efficient material, however, cannot disrupt the cost benefit of including an ERV within a HVAC system. As discussed hereinbefore, utilizing a ventilator to pre-condition the fresh air is an alternative to increasing the size of the HVAC system. Specifically, pre-conditioning the fresh air allows the system designers to utilize a design day having more moderate parameters, which, in turn, make possible the inclusion of smaller, less costly equipment. Such equipment will also consume less energy, thereby making it less expensive to operate. Hence, including an ERV within a HVAC system is perceived as a low cost method for increasing the system's overall operating efficiency. However, if the cost of a more efficient plate material significantly increases the first cost of the ERV, then including an ERV within a HVAC system decreases its financial benefit. Therefore, it is desirable that the plates within the plate-type heat exchanger be constructed of a low cost material, as well as a material that has the ability to effectively transfer latent heat.
Another alternative to increasing the plate material's ability to transfer latent heat is to pressurize the ERV because pressurizing the ERV increases the plate's ability to transfer latent heat from one air stream to the other by increasing the water concentration difference across the plate. A typical HVAC system, however, currently operates at about ambient pressure. Therefore, pressurizing the HVAC system and more particularly, the ERV, would require adding additional equipment, such as a compressor, to the HVAC system. Although pressurizing the ERV would increase its efficiency, adding the necessary equipment to pressurize the ERV would increase the HVAC system's overall cost. Again, including an ERV within a HVAC system is currently perceived as a low cost method for increasing its overall efficiency because doing so decreases the size and operating cost of the HVAC system. Pressurizing the HVAC system, alternatively, would only increase the size of such system by additional equipment, thereby eliminating the cost benefit of adding an ERV to an HVAC system.
What is needed is a plate-type heat exchanger wherein the plates are constructed of a cost effective material, other than paper, that is capable of transferring a larger percentage of the available latent heat in one air stream to the other air streams, while maintaining the ERV's ability to operate at about ambient pressure.